Download Benefits of a multiphase buck converter

Power Management
Texas Instruments Incorporated
Benefits of a multiphase buck converter
By David Baba
Applications Engineering Manager
Introduction
Single-phase buck controllers work well for
low-voltage converter applications with currents of up to approximately 25 A, but power
dissipation and efficiency start to become an
issue at higher currents. One suitable approach
is to use a multiphase buck controller. This
article briefly dis­cusses the benefits of using a
multiphase buck converter versus a singlephase converter and the value a multiphase
buck converter can provide when implemented.
Figure 1 shows a two-phase circuit. From
this circuit’s waveforms, shown in Figure 2, it is
clear that the phases are interleaved. Inter­leav­
ing reduces ripple currents at the input and
output. It also reduces hot spots on a printed
circuit board or a particular component. In
effect, a two-phase buck con­verter reduces the
RMS-current power dissipation in the FETs and
inductors by half. Interleaving also reduces
transitional losses.
Figure 1. Two-phase buck converter
VIN
Q1
Q1 Gate
Phase 1
Node
L1
Q2
Q2 Gate
VOUT
Multiphase
Controller
VIN
Q3
Q3 Gate
Phase 2
Node
L2
Q4
Q4 Gate
Output-filter consideration
Output ripple voltage
Ripple-current cancellation in the output-filter
stage results in a reduced ripple voltage across
the output capacitor compared to a single-phase
converter. This is another reason why a multiphase converter is preferred. Equations 1 and 2
calculate the percentage of ripple current canceled in each inductor.
m = D × Phases (1)
and
I Rip _ norm ( D) =
Figure 2. Node waveforms of phases 1 and 2
Phase 1
Phase 2
6
5
Switch-Node Voltage (V)
The output-filter requirements decrease in a
multiphase implementation due to the reduced
current in the power stage for each phase. For
a 40-A, two-phase solution, an average current
of only 20 A is delivered to each inductor.
Compared to a 40-A single-phase approach,
the inductance and inductor size are drastically
reduced because of lower average current and
lower saturation current.
4
3
2
1
0
1.628
1.629
1.630
1.631
1.632
1.633
1.634
Time (ms)
mp( D)   1 + mp( D)


 D − Phases  ×  Phases − D 
Phases ×
, (2)
(1 − D) × D
8
High-Performance Analog Products
www.ti.com/aaj
1Q 2012
Analog Applications Journal
Power Management
Texas Instruments Incorporated
Figure 3. Normalized capacitor ripple current as a function
of duty cycle
1
0.9
0.8
Normalized Ripple Current
where D is the duty cycle, IRip_norm is the normalized ripple current as a function of D, and
mp is the integer of m. Figure 3 plots these
equations. For example, using two phases at
a 20% duty cycle (D) yields a 25% reduction
in ripple current. The amount of ripple voltage the capacitor must tolerate is calculated
by multiplying the ripple current by the capacitor’s equivalent series resistance. Clearly, both
maximum current and voltage requirements
are reduced.
Figure 4 shows the simulation results for a
two-phase buck converter at a duty cycle of
25%. The inductor ripple current is 2.2 A, but
the output capacitor sees only 1.5 A due to
ripple-current cancellation. With a duty cycle
of 50% and two phases, the capacitor sees no
ripple current at all.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Load-transient performance
0
Load-transient performance is improved due
to the reduction of energy stored in each output inductor. The reduction in ripple voltage
as a result of current cancellation contributes
to minimal output-voltage overshoot and under­shoot
because many cycles will pass before the loop responds.
The lower the ripple current is, the less the perturbation
will be.
10
20
30
40
50
60
70
80
90
100
Duty Cycle, D (%)
Cancellation of input RMS ripple current
The input capacitors supply all the input current to the
buck converter if the input wire to the converter is inductive. These capacitors should be carefully selected to satisfy
the RMS-ripple-current requirements to ensure that they
Figure 4. Cancellation of inductor ripple current with D = 25%
Inductor Current (A)
6
4
Phase 1 Phase 2
2
0
–2
Output Capacitor Current (A)
–4
24
22
20
18
16
14
12
10
8
6
4
2
4.463
4.464
4.465
4.466
4.467
4.468
4.469
Time (ms)
9
Analog Applications Journal
1Q 2012
www.ti.com/aaj
High-Performance Analog Products
Power Management
Texas Instruments Incorporated
Figure 5. Normalized input RMS ripple current as a function
of duty cycle
Normalized Input RMS Ripple Current
0.250
0.225
0.200
0.175
0.150
0.125
0.100
0.075
0.050
0.025
0
0
10
20
30
40
50
60
70
80
90
100
Duty Cycle, D (%)
do not overheat. It is well understood that, for a singlephase converter with a duty cycle of 50%, the worst-case
input RMS ripple current is typically rated at 50% of the
output current. Figure 5 and Equation 3 indicate that, for
a two-phase solution, the worst-case RMS ripple current
occurs at duty cycles of 25 and 75% and is only 25% of the
output current.
I Input _ norm (D) =
mp(D)   mp(D) + 1


 D − Phases  ×  Phases − D  (3)
The value of a multiphase solution as compared to a
single-phase solution is clear. Less input capacitance can
be used to satisfy the RMS-ripple-current demands of the
buck stage.
Application example
The LM3754 high-power-density evaluation board delivers
1.2 V at 40 A from a 12-V input supply. The board is 2 × 2
inches, and the area covered by the components is 1.4 × 1.3
inches. The switching frequency of each phase is set to
300 kHz. Table 1 provides a summary of these and other
operating conditions. The components are placed on a
4-layer board, with 1 oz. of copper on all layers. Additional
pins are included on this board for remote sensing, and a
pin is used for margining the output voltage.
Because the LM3754 evaluation board is designed to
operate in high-power-density configurations, it utilizes the
optimized input capacitors to provide the reduced RMS
ripple current that is required. The evaluation board also
has a low ripple voltage and good transient perform­ance.
The board layout shown in the LM3754 application note1
should be followed as closely as possible. However, if this
Table 1. Operating conditions of LM3754
evaluation board
Input voltage
10.8 to 13.2 V
Output voltage
1.2 V ± 1%
Output current
40 A (max)
Switching frequency
300 kHz
Module size
2 × 2 inches
Circuit area
1.4 × 1.3 inches
Module height
0.5 inches
Air flow
200 LFM
Number of phases
2
is not possible, close attention should be paid to these
considerations. Several more layout considerations will
now be described, followed by the test results from a test
board using the LM3754. These results are presented in
Figures 6–11 on pages 12–13. They are typical of what one
can expect to achieve or even improve upon in making the
necessary modifications.
Layout considerations
High-current traces require enough copper to minimize
voltage drops and temperature rises. The general rule of
using a minimum of 7 mils per ampere was applied for the
2 oz. of copper used, and 14 mils per ampere for the inner
layers for the 1 oz. of copper used. The input capacitors of
each phase were placed as close as possible to the top
MOSFET drain and the bottom MOSFET source to ensure
minimal ground “bounce.”
10
High-Performance Analog Products
www.ti.com/aaj
1Q 2012
Analog Applications Journal
Power Management
Texas Instruments Incorporated
Signal components connected to the IC
Minimizing the switch node
All small-signal components that connected to the IC were
placed as close to it as possible. Decoupling capacitors for
VREF and VCC were also placed as close as possible to the
IC. The signal ground (SGND) was configured to ensure a
low-impedance path from the ground of the signal components to the ground of the IC.
To follow the common rules of keeping the switch-node
area as small as possible but large enough to carry high
currents, the switch node was built on multiple layers.
Because the small evaluation board essentially folds back
on itself from input to output, the switch node naturally sits
on the outer layer, and the IC sits directly underneath the
switch node. Therefore, it is essential to keep the switch
node well away from the sense lines and also from the IC.
Hence, the switch node was strategically placed facing
outwards toward the edge of the board.
SGND and PGND connections
Good layout techniques include a dedicated ground plane;
this board dedicated as much of inner-layer 2 as possible
for the ground plane. Vias and signal lines were strategically placed to avoid high-impedance points that could
pinch off wide copper areas. The power ground (PGND)
and SGND were kept separate, only connected to each
other at the ground plane (inner layer 2).
Gate drive
The designer should ensure that a differential pair of
traces is connected from the high-gate output to the top
MOSFET gate and the return, which is the switch node.
The distance between the controller and the MOSFET
should be as short as possible. The same procedure should
be followed for the LG and GND pins when the traces for
the low-side MOSFET are routed.
A differential pair of traces must also be routed from the
CSM and CS2 pins to the RC network located across the
output inductor. Notice in the layout in Reference 1 that,
in order to provide additional noise suppression, the filter
capacitor is split into two capacitors—one positioned by
the inductor and the other close to the IC. These sense
lines should not be run for long lengths in close proximity
to the switch node. If possible, they should be shielded by
using a ground plane.
Conclusion
There are a number of benefits to using multiphase buck
converters, such as higher efficiency from lower transitional losses; lower output ripple voltage; better transient
performance; and lower ripple-current-rating requirements
for the input capacitor. Some examples of multiphase buck
converters that can deliver the full benefits described
herein are the LM3754, LM5119, and LM25119 families.
Reference
1. Robert Sheehan and Michael Null, “LM3753/54 evaluation board,” National Semiconductor Corp., Application
Note 2021, Dec. 15, 2009 [Online]. Available: http://
www.national.com/an/AN/AN-2021.pdf
Related Web sites
power.ti.com
www.ti.com/product/partnumber
Replace partnumber with LM3754, LM5119, or LM25119
11
Analog Applications Journal
1Q 2012
www.ti.com/aaj
High-Performance Analog Products
Power Management
Test results
Texas Instruments Incorporated
Figure 6. Efficiency plot with 12-V input
90
VOUT = 1.2 V
Efficiency (%)
85
VOUT = 0.9 V
80
75
70
65
4
8
12
16
20
24
28
32
36
40
I OUT (A)
Figure 7. Power loss with 12-V input
9.65
8.65
Power Loss (W)
7.65
6.65
VOUT = 0.9 V
5.65
VOUT = 1.2 V
4.65
3.65
2.65
1.65
0.65
4
8
12
16
20
24
28
32
36
40
I OUT (A)
Figure 8. Switch-node voltages
VIN = 12 V, VOUT = 1.2 V at 40 A
12
High-Performance Analog Products
www.ti.com/aaj
1Q 2012
Analog Applications Journal
Power Management
Texas Instruments Incorporated
Figure 9. Output voltage ripple
VIN = 12 V, VOUT = 1.2 V at 40 A
Figure 10. Transient response: 20 µs with 10-A load
step (undershoot/overshoot ~ 27 mV)
Figure 11. VOUT start-up for 1.2-V output with 40-A load
13
Analog Applications Journal
1Q 2012
www.ti.com/aaj
High-Performance Analog Products
TI Worldwide Technical Support
Internet
TI Semiconductor Product Information Center
Home Page
support.ti.com
TI E2E™ Community Home Page
e2e.ti.com
Product Information Centers
Americas Phone
+1(972) 644-5580
Brazil
Phone
0800-891-2616
Mexico
Phone
0800-670-7544
Fax
Internet/Email
+1(972) 927-6377
support.ti.com/sc/pic/americas.htm
Europe, Middle East, and Africa
Phone
European Free Call
International
Russian Support
00800-ASK-TEXAS
(00800 275 83927)
+49 (0) 8161 80 2121
+7 (4) 95 98 10 701
Note: The European Free Call (Toll Free) number is not active in
all countries. If you have technical difficulty calling the free call
number, please use the international number above.
Fax
Internet
Direct Email
+(49) (0) 8161 80 2045
www.ti.com/asktexas
[email protected]
Japan
Phone
Fax
Domestic
International
Domestic
0120-92-3326
+81-3-3344-5317
0120-81-0036
Internet/Email International
Domestic
support.ti.com/sc/pic/japan.htm
www.tij.co.jp/pic
Asia
Phone
International
+91-80-41381665
Domestic
Toll-Free Number
Note: Toll-free numbers do not support
mobile and IP phones.
Australia
1-800-999-084
China
800-820-8682
Hong Kong
800-96-5941
India
1-800-425-7888
Indonesia
001-803-8861-1006
Korea
080-551-2804
Malaysia
1-800-80-3973
New Zealand
0800-446-934
Philippines
1-800-765-7404
Singapore
800-886-1028
Taiwan
0800-006800
Thailand
001-800-886-0010
Fax
+8621-23073686
[email protected] or [email protected]
Internet
support.ti.com/sc/pic/asia.htm
Important Notice: The products and services of Texas Instruments
Incorporated and its subsidiaries described herein are sold subject to TI’s
standard terms and conditions of sale. Customers are advised to obtain the
most current and complete information about TI products and services
before placing orders. TI assumes no liability for applications assistance,
customer’s applications or product designs, software performance, or
infringement of patents. The publication of information regarding any other
company’s products or services does not constitute TI’s approval, warranty
or endorsement thereof.
A011012
E2E is a trademark of Texas Instruments. All other trademarks are the property of
their respective owners.
© 2012 Texas Instruments Incorporated
SLYT449
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products
Applications
Audio
www.ti.com/audio
Automotive and Transportation www.ti.com/automotive
Amplifiers
amplifier.ti.com
Communications and Telecom www.ti.com/communications
Data Converters
dataconverter.ti.com
Computers and Peripherals
www.ti.com/computers
DLP® Products
www.dlp.com
Consumer Electronics
www.ti.com/consumer-apps
DSP
dsp.ti.com
Energy and Lighting
www.ti.com/energy
Clocks and Timers
www.ti.com/clocks
Industrial
www.ti.com/industrial
Interface
interface.ti.com
Medical
www.ti.com/medical
Logic
logic.ti.com
Security
www.ti.com/security
Power Mgmt
power.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Microcontrollers
microcontroller.ti.com
Video and Imaging
www.ti.com/video
RFID
www.ti-rfid.com
OMAP Mobile Processors
www.ti.com/omap
Wireless Connectivity
www.ti.com/wirelessconnectivity
TI E2E Community Home Page
e2e.ti.com
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2012, Texas Instruments Incorporated